In the case of a typical MR-imaging method, the object area which is to be examined, i.e. the “sample”, is disposed in a stationary magnetic field B0 and is subjected to a sequence of at least one electromagnetic high-frequency pulse (HF-pulse) of a selected frequency and to subsequent pulses of magnetic field gradients in various spatial directions, such that the HF-excitations cause echoes to occur which are detected as the MR-signal and provide information relating to the properties of the sample.
MRI tends to be the method of choice for noninvasive diagnosis of soft tissue disease in humans and has wide application in the medical field. Fast gradient technology has made high-resolution 3D imaging possible. However, high resolution MRI is primarily impeded by physiological movement, such as respiration or gross patient movement.
Diffusion weighted MR-Imaging has become more significant in recent years. By specifically selecting the type of MR-sequence and the amplitude and time relations of HF- and gradient pulses within the sequence, it is possible to ensure that the strength of the generated MR-signals or echoes depend considerably upon respectively determined, selected features of the sample. As a consequence, it is possible to generate an image, of which the contrast is “weighted” by the relevant feature.
With abdominal MR-imaging, images are affected by respiratory motion. Some MRI techniques, such as diffusion weighted MR-imaging, have a very low signal to noise ratio (SNR) on a single signal measurement. As a result, multiple measurements are usually performed in respect of a single slice location, so as to generate low SNR measurements, and these measurements are then averaged to create a single high SNR image. However, this type of averaging operation gives rise to blurring on moving objects because each measurement is performed at different timings of respiratory motion (i.e. scan location is shifted for each single measurement), if no synchronization method is used.
Respiratory triggering is one such synchronization method for ensuring that each measurement is performed on the same timing of the respiratory motion. As a result, every measurement is performed when the moving object is in the same position and this can avoid blurring due to motion. However, this technique has two problems associated therewith. One is that it restricts the timing of image acquisition in that there is a wait time required before and after each measurement is performed to wait for the moving object to return to exactly the same location during the motion cycle. This can lengthen total scan time by two or three times. Another problem is scan failure caused by a change in respiratory activity of the patient. A patient having a MRI scan can fall asleep during examination. This makes their breath very shallow such that the MR scanner continually fails to catch the trigger from the signal of a respiratory motion detecting device.
Navigator echo is another motion compensation technique, which detects motion prior to data acquisition and modifies data acquisition accordingly. Navigator techniques use image space navigator echoes for detecting motion during image data acquisition. Physiological motion causes global displacement in the navigator echo and results in a shift of the image space navigator echo compared to an image space reference navigator echo. The accuracy in extracting motion information from navigator echoes is crucial to the effectiveness of the navigator technique, and one such technique is described in US Patent Application Publication No. US2003/0117136A1. Effectively, in all navigator echo techniques, displacement of the moving object is measured prior to an imaging scan for each measurement and the imaging location is shifted based on the measured displacement. As a result, the same location of the object is imaged during multiple measurements and this avoids blurring due to motion. In contrast to the respiratory triggering technique described above, the navigator echo technique does not require a wait time to synchronize the imaging timing to respiratory motion. Therefore, the total scan time tends to be much shorter than for the respiratory triggering method.
However, one problem with the navigator echo technique arises when the imaging region contains both static and moving objects (for example, the liver and the spine). If the navigator echo method is applied an image focused on the moving object, the static object will be blurred. On the other hand, for an examination such as tumor screening, both moving organs and static objects should be of similar image quality. In that case, an MR user has to perform two scans, one with and one without navigator echo, which of course extends overall scan time.